Electronic circuit assemblies are often required to be capable of surviving in hostile operating environments, such as those commonly found in the automotive and aerospace industries. Such assemblies often employ surface mount integrated circuit (IC) packages, which can generally be characterized as being electrically and mechanically attached to the substrate of the assembly with a number of leads that are soldered to both the IC package and conductors formed on the substrate. The solder joints are subject to thermal stresses as a result of temperature fluctuations in the assembly's working environment and differences in coefficients of thermal expansion of the various materials used in the construction of the electronic assembly. These thermal stresses can potentially cause irreversible damage to the solder joints.
The above is further aggravated by the use of a conformal coating, which is widely used in the electronics industry to form a protective barrier layer on the circuit board. Conformal coatings are generally formulated from polymeric materials of the silicone, acrylic, urethane and epoxy families, and serve to protect the electronic assembly from moisture and dirt, as well as make the IC packages mounted to the circuit board more resistant to vibration. However, the leads support the IC packages above the surface of the substrate, such that a void is present between surface mount IC packages and the substrate. Consequently, during the conformal coating process, there is a tendency for the conformal coating material to flow beneath the IC packages. As such, the conformal coating tends to bridge the gap between the IC packages and the substrate, which induces stresses that are particularly damaging due to the vast difference in coefficients of thermal expansion between the conformal coating material and the solder and lead materials. Stresses may also be induced as a result of the conformal coating material swelling in reaction to contact with a solvent. Additional factors which effect the solder joint stress include the conformal coating thickness, its bulk modulus of elasticity, the physical characteristics of the component and its leads, the size of the gap between the component and the board, and the coefficient of thermal expansion of the component and the board.
As a result of the above, the expected life of the solder joints may be significantly decreased, at times on the order of up to a 90 percent loss in expected life. Notably, current integrated circuit packaging trends are toward IC packages with lower standoffs and less compliant leads, therefore resulting in packages which even more prone to damage.
In the past, masking materials have been used to form a permanent, nonremovable barrier around the perimeter of surface mount IC packages in order to prevent the conformal coating from flowing between the package and the substrate. Ultraviolet-curing (UV-curing) and room temperature vulcanizing (RTV) silicones have been used for this purpose, but with certain significant disadvantages. For example, the cost of employing UV-curing silicones is relatively high, including the raw material costs, the cost of the UV-curing equipment, and the maintenance and operating costs of the equipment. RTV silicones are also relatively expensive, and are somewhat difficult to use in a continuous production line operating at levels necessary in the automotive industry. For example, as applied, RTV silicones require about three to about seven days to fully cure. At viscosities on the order of about 10,000 cps, RTV silicones tend to flow readily, such that they will rapidly bridge the gap between the IC package and the substrate. RTV silicones applied at higher viscosities, as high as 100,000 cps or more, are also likely to extrude between the leads and beneath the IC package if the dispensing nozzle contacts the leads. Additional disadvantages of RTV silicones include their poor compatibility with coating materials other than silicones, and the release of condensation reaction by-products, such as methanol, ethanol and acetone, which have maximum allowable exposure limits.
From the above, it can be seen that it would be desirable if a method were available which could prevent the ingress of a conformal coating material into the void between a surface mount IC package and the substrate of an electronic circuit assembly. More specifically, such a method would have minimal impact on the processing and costs of such electronic circuit assemblies, and would not require the use of a solvent so as to avoid the environmental and safety hazards associated with solvents.